The invention relates to sewing machines, and more particularly relates to automatic sewing machines which are used in commercial or industrial applications to sew, e.g., label seams, collar seams and pocket seams.
These seams are examples of seams which should be neatly sewn because of the attention paid to them. For example, in the case of a shirt pocket, the corners of the seams which attach the pocket to the shirt should be equally spaced from the edge of the pocket. If one corner is closer to this edge, the pocket workmanship appears sloppy.
Various measures have been proposed to end a seam at the desired location. For example, it is known to abruptly cease feeding the work when the needle is positioned directly over the desired seam end point, and it is also known to vary the pitch of the last stitch in the seam and to operate the reverse mechanism of the machine in order to terminate the seam at the desired location.
These known measures are not entirely satisfactory. This is because the seam will have normally sized stitches at its beginning and middle, and will have a much shorter stitch at its end, with an abrupt change between these stitches of different lengths. This abrupt change detracts from the appearance of e.g. the pocket or collar seam.
One object of the invention is to provide a device which will automatically end a seam at a predetermined location in a neat and attractive manner.
Another object is to provide a device which will make the difference between the length of stitches at the end of the seam and the length of stitches at the beginning and middle of the seam less conspicuous.
Still another object is to provide a device which is reliable and manufacturable at reasonable cost.
A further object is, in general, to improve on known devices of this type.
In accordance with the invention, there is provided a sewing machine which sews the last n stitches of a seam in such a manner that the seam ends at a predetermined point and the n stitches have substantially equal lengths. This is accomplished by monitoring the position of the work, e.g. the pocket or the collar, as by monitoring the work for detection of its trailing edge. When a predetermined spaced relation exists between the sewing needle and the desired endpoint of the seam, the number of stitches needed to sew to the end of the seam is computed, and stitch pitch is varied as the seam is completed. The variation is carried out in such a manner that the last n stitches in the seam are equally long and end at the desired end point.
In preferred embodiments, the lengths of the last two stitches of the seam are substantially equalized and the pitch of the previous stitches is at its maximum value as established by the operator. Advantageously, a pulse generator generates pulses in synchronism with the main shaft of the sewing machine and the pulses are counted during those times when the feed dog is feeding the work toward the needle. When the trailing edge of the work is detected, the number of pulses counted at the time of detection is used to determine the pitch of the last two stitches in the seam.